Diocletian's Tetrarchy

Timeline

1. 293–305Nicomedia · BithyniaDiocletian and Maximian served as Augusti of the East and West, with Galerius and Constantius as Caesars, forming a structure of two Augusti and two Caesars, with regular abdication and rotation of power to stabilize imperial rule.
2. c. 293–312Provinces of the Roman Empire · MediterraneanDiocletian divided the empire into about twelve dioceses, introduced a combined land-and-head tax system (capitatio-iugatio), and strengthened the military frontier system, complementing the Tetrarchy.
3. 305–324Milvian Bridge—Chrysopolis · ItalyAfter Diocletian's abdication, succession disputes among the Tetrarchs sparked civil wars; Constantine defeated Licinius in 324 to reunify the empire, formally ending the Tetrarchy, though the East-West division had become irreversible.
4. 324–500Constantinople—Milan · MediterraneanThough the Tetrarchy was abolished, Diocletian's administrative divisions, court ceremonial, military organization, and multi-capital system were inherited by the Late Roman Empire and Byzantium; the diverging political centers of East and West continued to shape the fifth century.